Such street grids for surface drainage are known and as a rule are installed in such a way that the surface of the frame and the surface of the grating are flush with the upper edge of the paved gully. Water flowing toward the street grid within the gully, derived for example from rain or melting snow, then enters the street drainage system by passing over the frame and through the grating. Another embodiment of a street grid is conceived such that the water flowing toward it enters primarily between the frame and the grating, and is additionally diverted so as to enter by way of the grating only if the influx increases.
In the case of existing street grids in channel form, in practice it can happen that after an input channel has been completed, for instance a pavement channel or one made of gully stones, it can settle to a different extent than the street grid. Because of unpredictable features of the ground structure, overloading or design deficiencies, such differences are to some extent unavoidable. If as a result the upper edge of the grid frame is higher than the bottom of the channel, the water flowing in through the channel is no longer completely drained away, and residual puddles are produced in the channel and in the adjacent surfaces because the water builds up against the upper edge of the grid frame.
These residual puddles are problematic and dangerous both for pedestrians and also for traffic flow. Furthermore, they also result in damage to the drainage system in general. If the water in the puddles freezes, very large pressures are generated especially at the junctions between channel and adjacent surface, for example a sidewalk; these forces can enlarge the gaps at the junctions and thus loosen the seating of the whole drainage channel, so that eventually the structure is destroyed. Because of these subsidence hollows solid materials contained in the water, such as leaves, pollen or loose chippings, can no longer be completely transported away and are deposited in front of the edge of the street-grid frame. Especially in the case of grids such that the water primarily flows away between frame and grating, these suspended masses rapidly cause blockages that in this case, because of their awkward positions, can be cleared only by hand or with high pressure. A self-cleaning process is hardly to be expected here.